A secondary battery is a chemical battery that can be used semipermanently by continuously repeating charge and discharge using an electrochemical reaction, and can be classified into a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium secondary battery. Among them, a lithium secondary battery has a higher voltage and better energy density characteristics than the others, and thus is taking the lead in the secondary battery market. Also, depending on the type of an electrolyte, a lithium secondary battery can be divided into a lithium ion secondary battery using a liquid electrolyte and a lithium ion polymer secondary battery using a solid electrolyte.
A lithium secondary battery includes a cathode, an anode, an electrolyte, and a separator, and among them, the separator plays a role of separating the cathode from the anode to electrically isolate the cathode and the anode and to improve the transfer of lithium ions or permeability based on a high porosity to increase ionic conductivity. As a polymer substrate of a generally used separator, a polyolefin-based polymer having advantageous properties for pore formation, chemical resistance, and excellent mechanical and thermal properties, such as polyethylene (PE), polypropylene (PP), and the like, is mainly used.
A separator for a lithium secondary battery requires characteristics such as excellent permeability, low thermal shrinkage, high puncture strength, and the like; however, with the advancement of a high-capacity high-output battery, attempts to improve permeability are continuously being conducted. To manufacture a porous separator from polyolefin, a wet process that mixes polyolefin and a pore forming agent at high temperature, extrudes, and stretches, and subsequently, extracts the pore forming agent have been used. However, to improve permeability of a separator manufactured through the wet process, a way of increasing an amount of the pore forming agent, for example, a diluent, a plasticizer, and the like, have been used; however, as the content of the pore forming agent increases, stability of extrusion molding greatly reduces, difficulties occur since alterations have to be made to various process conditions including extrusion conditions, and environmental issues are raised due to a great amount of pore forming agents and solvents.
Meanwhile, in contrast to a wet process using a solvent, a dry process without using a solvent enables mass production of a wide film and is more environmentally friendly than the wet process because there is no need for a solvent. However, a stretching process has a drawback in that a possibility of a short circuit occurring is high due to the tendency of a film shrinking in a direction opposite of its stretched direction.
Accordingly, a film having passed through a stretching process has significant improvements in terms of mechanical properties such as a tensile strength in a stretched direction or a more stretched direction, but has a relatively low strength in a direction perpendicular to its stretched direction or a less stretched direction, so that the film may be torn out, resulting in a short circuit. Also, an internal short circuit may occur when the separator shrinks excessively due to increased temperature caused by overcharge or other reasons.
Accordingly, there is still a demand for a porous separator in which a stretched film has improved strength in a stretched direction as well as a direction opposite to its stretched direction.